Revert PIPELINE to test_mode: true — SIGSEGV in production mode

Production condensation (test_mode: false) caused SIGSEGV in TID within
~80s of startup despite #260 ring improvements + tombstone guard. Reverted
immediately. Root cause investigation required before re-enabling.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

rust_core/src/membrane.rs

@@ -492,7 +492,7 @@ static MEMBRANE: std::sync::LazyLock<Mutex<MembraneState>> =
 ///   3. recently_freed tombstone (5s): blocks reused addresses after free processing
 static PIPELINE: std::sync::LazyLock<Mutex<Pipeline>> =
     std::sync::LazyLock::new(|| Mutex::new(Pipeline::new(PipelineConfig {
-        test_mode: false,
+        test_mode: true,  // REVERTED 2026-06-21: SIGSEGV in production mode
         ..PipelineConfig::default()
     })));